11/8/2012 3:30 PM PST

"Modern Family" star Ariel Winter's mother insists she did NOT abuse her daughter -- claiming 14-year-old Ariel is making up stories ... because her mother wouldn't let her engage in a sexual act with her 18-year-old boyfriend.

Ariel's mom Chrystal Workman released a statement today, saying ... “I love my daughter very much. I would never abuse her in anyway and I have always tried my best to always protect her and do what is right for her."

She explained, "My fourteen-year-old daughter has been dating an eighteen-and-a-half-year-old man, and when I caught them engaging in behavior that I feel my daughter is too young mentally and physically to understand and fully grasp I put a stop to it immediately."

Chrystal doesn't identify the boyfriend ... but says she filed a report with the LA County Sheriff's Department "because legally he is an adult and if he wants to engage in adult behavior with a minor than [sic] he should also be ready to suffer adult consequences for his actions."

According to the incident report, Chrystal walked in on Ariel and the boyfriend in bed, in the guest room of her home ... on Sept. 24 at around 10pm.

Law enforcement sources tell TMZ the Sheriff's Special Victims' Bureau does have an open investigation into Chrystal's allegations.

However, TMZ has learned ... Chrystal's report wasn't filed until 2 weeks after the alleged incident -- and 5 days AFTER she lost custody of Ariel.

TMZ broke the story ... the 14-year-old actress, who plays Alex Dunphy on 'MF', was removed from her mother's care by a judge in the wake of serious allegations of physical and emotional abuse.

Ariel's older sister, Shanelle, now has temporary guardianship of the actress.

Chrystal claims Shanelle only came "out of the woodwork once it was announced that the kids on the show got a big raise recently."